This invention relates to distance measurement, particularly an optical distance measurement system.
One common approach to measuring distance to a far object is to measure the time of flight of a pulse of light from the measuring system to the far object and back again, and then to calculate the distance to the far object based upon the speed of light. Systems employing this method commonly employ a laser to generate the light pulse and so are known generically as xe2x80x9claser range findersxe2x80x9d (LRF""s) or xe2x80x9clight detection and rangingxe2x80x9d (LiDAR) systems. Typical applications are the measurement of altitude, target range or distance for survey applications in civil engineering and metrology. LRF""s may be built as stand alone hand held units or embodied in larger systems.
A known LRF is shown in FIG. 1 and comprises a laser 1, an optical transmission system 2, an optical reception system 3, a light sensitive detector 4, pulse detection circuitry 5, and timing calculation and display electronics 6.
In operation, the user initiates a measurement of range using input 7, which causes a laser fire pulse to be sent to the laser 1 and the laser to emit a pulse of light at time T0 as represented by the plot 10. This pulse is focussed by the transmission optics 2 and travels to the remote object 8 where it is reflected. The receiving optics 3 collects a portion of the reflected light pulse illustrated as plot 12 and focuses the energy onto the light sensitive detector 4. The detector 4 converts the received light pulse into an electrical signal and the pulse detector 5 discriminates against any electrical noise generated by the light sensitive detector to provide a clean, logic level pulse from the incoming light detector signal at time T1.
This pulse is passed to timing calculation and display electronics 6 which calculates and displays the range to the remote object based upon the time of flight of the laser pulse (T1-T0) and the speed of light (c) in the intervening medium.
Often there are multiple pulses apparent within the reflected signal captured by the detector 4 due to reflections from a number of different objects (e.g. vegetation) in the path of the light pulse or variations in the refractive index of the intervening atmosphere. LiDAR systems, instead of using a simple pulse discrimination system 5, apply further signal processing and analysis to the signal output by detector 4 to calculate the position and strength of these additional reflections and hence enable various characteristics of the intervening objects or atmosphere to be studied.
To reduce cost, some LRF devices employ a single optical system with an optical beam splitter to separate the transmitted and reflected pulses.
The maximum range that can be measured by an LRF or LiDAR system is determined by the point at which the LRF can no longer discriminate between the incoming reflected pulse and any background illumination or effects inherent in the optical detector such as e.g. thermally generated dark current and shot noise.
Because of the losses in transmission and reflection, to achieve ranges of more than a few kilometers requires laser technologies such as Nd:YAG or Erbium:Glass which are relatively expensive. Lower cost systems have been built using solid state laser diodes but because the energy in each transmitted pulse is relatively low their range is limited to a few hundred meters.
Some systems extend the range by sending many (N=hundreds or thousands) pulses and summing the reflected signals to improve discrimination against the uncorrelated detector noise. Using this technique an LRF using solid state laser diodes can achieve a range of up to 2-4 Km. However, this process only provides a N improvement in discrimination at best. In addition, because a pulse cannot be transmitted until the reflection of the previous pulse has been detected to avoid ambiguity, at long ranges the pulse repetition rate is limited. For example to send and receive a pulse to a remote object at 5 Km takes xcx9c30 xcexcS and so to collect 1000 samples takes 0.03 S. In practice, it is found that over this time period slight movements in the line of sight or the remote object can substantially reduce the advantages of summing the received pulses.
There is also a trade off between the range and the light transmitting and light gathering capabilities of the optical systems used. Wide aperture optical systems will improve range but increase size and cost.
To overcome the disadvantages of these systems, alternative approaches have been developed.
One example is the system described in GB 1 585 054. In this system the output of a Carbon dioxide laser is passed through an acousto-optical modulator and output. Received infra-red signals are detected, and an electronic circuit using surface acoustic wave devices is provided that can determine the range and velocity of a target.
One particularly effective embodiment of the technique can be achieved using a signal known as the Maximal Length Sequence (MLS). This is a family of pseudo random noise binary signal (PRBS) which are typically generated using a digital shift register whose input is generated from appropriate feedback taps. The use of such a sequence is described in GB 1 585 054.
The maximal length sequence is the pseudo random noise sequence with the longest period which can be generated with a shift register of r sections. It has a length N=2xe2x80x2xe2x88x921 shift register clock cycles and has good auto-correlation properties as the auto-correlation function has only two values; either xe2x88x921/N or a peak of 1.0 at the point of correlation.
FIG. 2 illustrates one example of a maximal length sequence generated by a four stage shift register 20. Alternative length sequences can be generated by using longer shift registers with the appropriate feedback taps.
This approach may also be combined with averaging techniques to improve the signal to noise ratio and hence range further.
Another document describing a similar approach is DE 199 48 803 which describes a rangefinder. A maximum length sequence (MLS) is transmitted and correlated with a received reflected signal. The MLS is a good choice because its binary nature allows efficient modulation of laser diodes. In addition, because the signal only takes values of +1 and xe2x88x921, the cross-correlation can be computed simply only additions and subtractions, without the need for multiplications.
A further difficulty is that the distance precision is limited by the sample rate of the analogue to digital converter. For example, if the sample rate is 33 MHz, then the smallest time increment which can be measured is xcx9c30 nS which equates to a distance precision of xcx9c5 m. This is insufficient for many applications. To overcome this problem, the sample rate can be increased, but this increases system cost because more expensive components and more sophisticated circuitry are needed.
In the apparatus of DE 19948803 a controllable delay line is provided between the transmitted signal and the cross correlator; the length of the delay introduced by the delay line is controlled by the timing electronics. In operation, successive MLS signals are transmitted and the delay line is adjusted in small steps until the correlation peak is maximised. The total time of flight is calculated from the whole number of MLS clock cycles plus the small delay added by the delay line at which the correlation peak is maximised. In DE 199 49 803 the delay step size is set equal to one fifth of the MLS clock sample frequency and so the precision of the time and distance measurement is increased by a factor of five. A major disadvantage of this technique is that the total measurement time is increased by the number of steps required to find the correlation peak. This is problematic in many applications:
a) in low power, hand held applications transmitting more MLS signals wastes power, reducing battery life;
b) in covert applications, increasing the number of MLS cycles transmitted increases the probability of detection; and
c) in real time measurement applications increasing the number of MLS cycles reduces the number of distance measurements which can be made in a given time.
A further point is that in order to fully calculate a cross correlation coefficient a significant level of computation would be required. For example, an MLS signal of order 10 comprises 1023 clock cycles so to fully calculate the cross-correlation of the modulation and received signals would require 10232 or 1046529 operations.
To overcome this difficulty, the design described in DE 199 49 803 uses a comparator to convert the received signal into a single bit signal. This enables the cross-correlation function to be computed using an exclusive OR gate and shift register combination. Whilst this simplifies the cross-correlation calculation the substantial loss of signal information incurred with the one bit digitisation much reduces the effectiveness of the technique, particularly where the reflected signal amplitude is equal to or less than the noise from the detector and the ambient.
The proposed invention seeks to overcome these problems.
According to the invention there is provided an optical distance measuring equipment comprising
a signal source for supplying a modulation signal,
a transmission system connected to the signal source for transmitting a transmitted optical signal modulated by the modulation signal,
a reception system for receiving a received optical signal which is a reflected and delayed version of the transmitted signal,
and a cross-correlator arranged to carry out the steps of
determining, at a coarse resolution, the time delay of the modulation signal needed to maximise the correlation between the time delayed modulation signal and the received signal,
determining at a finer resolution than the coarse resolution, the correlation between the modulation signal and the received signal as a function of the time delay of the modulation signal with respect to the received signal in a time delay range around the determined time delay, and
outputting a measure of distance calculated from the time delay of the modulation signal needed to maximise the correlation between the time delayed modulation signal and the received signal.
As will be explained in more detail below, the determination of a coarse value of the time delay and the use of the coarse value to calculate more accurately the time delay can greatly reduce the number of calculations that are required.
Furthermore, good accuracy can be obtained without the need to transmit repeated MLS signals.
The output measure can be the distance itself, in duly convenient units, or alternatively a measure of distance such as the calculated time delay, or any other measure such as area or volume including or based on the distance.
The simplified calculation permits the use of a multiple bit analogue to digital converter for digitising the received signal and outputting a multiple bit output for each clock cycle of the analogue to digital converter. In DE 199 49 803 on the other hand the signal is digitised using a comparator, effectively a one bit digital to analogue converter. It would be very difficult in the arrangement of DE 199 49 803 to calculate cross-correlations using multiple bit precisions, since the number of calculations used would be very high. The use of the coarse and fine correlation calculations in the present invention reduces the calculation load and accordingly permits the use of a higher resolution representation of the received signal.
Indeed, somewhat surprisingly, the time delay output may be determined on a resolution that is even finer than the time resolution used in either of the coarse and fine determinations. This may be done by providing means for calculating the parameters of a straight line fit to the correlation output by the fine cross-correlator as a function of time delay in the time shift intervals before the peak time shift, for calculating the parameters of a straight line fit to the correlation output by the fine cross-correlator as a function of time delay in the time shift intervals after the peak time shift, and for calculating the peak time shift from the fitted parameters of straight line fits.
In embodiments, the optical distance measuring equipment may comprise a coarse cross-correlator for coarsely determining the time delay of the modulation signal needed to maximise the correlation between the time delayed modulation signal and the received signal, and a fine cross-correlator for calculating the correlation between the modulation signal and the received signal as a function of the time delay of the modulation signal with respect to the received signal in a time delay range around the time shift determined by the coarse cross-correlator.
Alternatively, the coarse and fine calculations can be carried out in a single digital signal processor and code programming the processor to carry out the coarse and fine time delay determinations sequentially.
The coarse cross correlator may be clocked at a first frequency determining the coarse resolution and the fine cross-correlator may be clocked at a higher second frequency.
Because the coarse cross-correlation can be computed quickly, the coarse correlation can be periodically calculated on the signal stored in the averaging memory (52). When the coarse correlation shows that an adequate signal to noise ratio has been achieved the fine correlation can be performed to give a final, accurate distance result. In this way, the laser energy needed to measure any particular distance is always kept to a minimum which is of benefit in battery powered applications and maximises eye safety in any application.
The signal source may be clocked at a sub-multiple of the second clock frequency different to the first clock frequency. This can improve the detection of the coarse time delay when the signal to noise ratio is poor. Indeed, the first, second and signal source clock frequencies may be adjustable for allowing adaptive optimisation of performance for different applications.
The signal source may produce a digital modulation signal clocked at a frequency lower than that of a clock input to the cross-correlator. The digital modulation signal may be a maximum length sequence. Such sequences give a useful, known cross-correlation function with a triangular peak.
The analogue to digital converter may be clocked at the higher second frequency used for the fine determination of the time delay.
In another aspect, the invention relates to a method of optically measuring distance including
supplying a modulation signal,
transmitting a transmitted optical signal modulated by the modulation signal,
receiving a received optical signal which is a reflected and delayed version of the transmitted signal,
coarsely determining the time delay of the modulation signal needed to maximise the correlation between the time-delayed modulation signal and the received signal, and
calculating at a finer resolution the correlation between the modulation signal and the received signal as a function of the time delay of the modulation signal with respect to the received signal in a time delay range around the time shift determined by the coarse cross-correlator to give a measure of distance.
The step of coarsely determining the time delay may be carried out at a first clock frequency and the step of calculating the correlation at a second higher clock frequency.
The signal source may be clocked at a sub-multiple of the second clock frequency different to the first clock frequency. The signal source may produce a digital modulation signal clocked at the first frequency. The digital modulation signal may be a maximum length sequence.
The received signal may be digitised at a multiple bit resolution.
The analogue to digital converter may be clocked at the higher second frequency.
To enhance accuracy, the method may further comprise
calculating the parameters of a straight line fit to the correlation output by the fine cross-correlator as a function of time delay in the time shift intervals before the peak time shift,
calculating the parameters of a straight line fit to the correlation output by the fine cross-correlator as a function of time delay in the time shift intervals after the peak time shift, and
calculating the peak time shift from the fitted parameters of straight line fits.
The input on the coarse cross-correlator may be filtered through a low-pass filter.